In Italy the production of solid urban refuse currently increases by about 1% per year and is today estimated at 18/20 million tonnes per year, of which 1.4 million (about 7.5%) are plastics materials.
Recycling this large quantity of material would enable its salvage value to be recovered, leading to undoubted economic benefits, a reduction in the global volume of refuse and a reduction in the consumption of virgin materials. Its energy content could also be recovered by, at the end of its second cycle of life, incinerating it in a furnace using a suitable heat recovery method. A solution of wide impact, in terms of its applicability to different situations and to different treatable quantities, is the reuse of recycled polyethyleneterephthalate (PET), originating from the sorted collection of liquid containers, for the production of moulded articles by injection, extrusion or blow moulding.
From the literature, processes are known for recovering PET contained in mixtures of plastics materials originating from the sorted collection of solid urban refuse.
Published European patent application 291,959 describes a method for separating articles of polyolefin and/or PET construction from polyvinylchloride (PVC) articles by irradiating these articles with electromagnetic radiation. The separation is based on the greater capacity of chlorinated resins, such as PVC, to absorb the electromagnetic radiation than other non-chlorinated resins, with deviation of the articles to different destinations on the basis of the different absorption of the electromagnetic absorption. Published European patent application 469,904 describes a method for separating heterogeneous plastics materials into two homogeneous fractions, one consisting essentially on PET and the other one PVC, which comprises:
1) recovering a heavy fraction, consisting essentially of PET and PVC, by flotation in water; PA1 2) treating the heavy fraction with acetone, which swells the PVC while leaving the PET unaltered; PA1 3) stripping the absorbed acetone, which is recycled; PA1 4) separating the PET from the swollen PVC by flotation in water or another liquid of greater density than water, PA1 a) reacting the recycled PET at elevated temperature with at least one oxazoline of general formula: ##STR2## where R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which can be the same or different, represent a hydrogen atom, a halogen such as chlorine, or an alkyl, cycloalkyl, aryl, alkylaryl, alkoxy or carboxyalkyl radical containing from 1 to 20 carbon atoms, and Z represents a trivalent linear, branched or cyclic aliphatic radical containing from 1 to 16 carbon atoms and possibly heteroatoms such as oxygen, sulphur or nitrogen, or Z represents an aromatic or alkylaromatic radical containing from 6 to 20 carbon atoms; PA1 b) cooling the reaction product obtained to ambient temperature. The compounds of general formula (I) are known products and can be prepared as described in R. H. Wiley, L. L. Bennet, "Chemical Review", 44, 447 (1949). PA1 i) mixing the recycled PET in the dry state with compounds of type (I) preceded by drying the polymer under vacuum at a temperature of between 80.degree. and 150.degree. C.; PA1 ii) heating to elevated temperature, above the polymer melting point, until the reaction has taken place; PA1 iii) cooling the product obtained to ambient temperature.
However, the PET polymer fractions obtained by the aforesaid methods still contain substantial traces of PVC or other impurities (traces of paper, aluminium, adhesives etc.) which cause uncontrolled degradation of the polymer during the subsequent transformation stages in the molten state, with consequent undesirable formation of carboxyl groups and a lowering of the molecular weight. Further treatment aimed at increasing the PET purity could result in an increase in production cost to the extent of making the recovery process uneconomical. An alternative could be to regrade the recycled PET by reactive modification, preferably in the molten state,
From the literature, additives are known which facilitate chain extension reactions of thermoplastic polyesters to give polymers of higher molecular weight and a reduced content of carboxyl groups.
Published Japanese patent applications 82-49,616 and 57-161,122 describe the use of cyclic bis(imino)ethers in chain extension reactions on PET and polyarylates.
Published Japanese patent applications 60-161,427 and 60-163,921 describe the use of 2,2'-(m-phenylene)-bis-2-oxazoline as a chain extender for PET, whereas in published Japanese patent application 63-313 this reagent is used in conjunction with 2,2'-bis-2-oxazoline.
However, as demonstrated hereinafter, in the case of recycled PET the stated additives allow only partially satisfactory results to be obtained. In this respect it has been verified that the rheological properties of these materials when in the molten state, in particular the apparent viscosity, tend to diminish with time, with evident repercussion on the processability of the polymer.